1. Field of the Invention
The present invention relates generally to telephony over a local IP (Internet Protocol) network, and in particular, to a system and method for conducting a telephone call for a plurality of users over a local IP network to which a single IP address is assigned. The present application is based on Korean Patent Application No. 14102/2000 filed on Mar. 20, 2000, which is incorporated herein by reference.
2. Description of the Related Art
FIG. 1 is a block diagram of a conventional system for telephony over an IP network, FIG. 2 illustrates protocols for telephony in a terminal of the conventional system, FIG. 3 is a signal flow diagram for discovering a gatekeeper in the conventional system, and FIG. 4 is a signal flow diagram for a call process in the conventional system.
Referring to FIG. 1, a VOIP (Voice Over Internet Protocol)-based telephone call is conducted over an IP network 110 according to the ITU-T (International Telecommunication Union-Telecommunication Sector) Recommendation H.323.
A gatekeeper 120 acts as a server for conversion between a telephone number input by an end user and an IP address, and provides registration, authentication, and RAS (Registration Admission Status) management. Computers 150 and 151 are assigned to unique IP addresses and connected to the IP network 110. The IP network 110 is connected to the PSTN (Public Switched Telecommunication Network) 180 and 181 via gateways 190 and 191, respectively. An I-PHONE 160 is an Internet phone for an Internet call.
H.323 provides protocols as shown in FIG. 2. According to H.323, a communication is conducted by TCP (Transmission Control Protocol) 210 or UDP (User Data Protocol) 220 based on IP (Internet Protocol) 200.
To attempt connection of an initial call, call signaling is performed according to H.225 (Q.931) 211, call control according to H.245 212, and data transmission according to T.120 213. H.225, H.245, and T.120 are based on TCP 210.
After call connection, G.7XX 221 controls voice transmission and H.26X 222 controls video transmission. G.7XX and H.26X are processed by RTP (Real Time Protocol) 223 for real-time transmission, and RTCP (real-time transport control protocol) 224 controls an RTP channel. G.7XX, H.26X, RTP, and RTCP are based on UDP 220. TCP requests an acknowledgment and UDP transmits data regardless of the acknowledgment. UDP 220 processes voice or video data because its slight loss is negligible to a user in data recovery.
Referring to FIG. 3, a description will be made of an operation of discovering the gatekeeper 120 for a telephone call over the IP network 110 in the computer 150.
In step 300, the computer 150 transmits a gatekeeper request message GRQ using a multicast address by UDP in order to discover the gatekeeper 120. The gatekeeper 120 transmits a gatekeeper confirm message GCF to the computer 150 as a response to the gatekeeper request message GRQ in step 310. The gatekeeper confirm message, GCF contains the IP address of the gatekeeper 120. Upon receipt of the gatekeeper confirm message GCF, the computer 150 transmits a registration request message RRQ to the gatekeeper 120, thereby registering a Q.931 transport address (IP address+TCP port number) and an alias address of the computer 150 and the IP address of a destination (i.e., a receiving computer). The alias address is registered when a local IP network is assigned to a plurality of IP addresses. Such a local IP network dynamically allocates the IP addresses to computers connected to the local IP network upon request of Internet access and a gatekeeper registers the IP addresses assigned to the computers.
When registration is completed, the gatekeeper 120 transmits a registration confirm message RCF to the computer 150 in step 330. For this purpose, the ITU-T defines UDP and TCP channels as shown in Table 1 below.
TABLE 1GATEKEEPER DISCOVERY MULTICAST ADDRESS224.0.1.41GATEKEEPER UDP DISCOVERY PORT1718GATEKEEPER UDP REGISTRATION AND STATUS1719PORTENDPOINT TCP CALL SIGNALLING PORT1720(in the case of direct call signaling to an endpoint without agatekeeper)
A procedure subsequent to the registration will be described with reference to FIG. 4.
Referring to FIG. 4, the computer 150 transmits an admission request message ARQ to the gatekeeper 120 in step 400 and the gatekeeper 120 transmits an admission confirm message ACF to the computer 150 in response to the admission request message ARQ as long as the receiving computer is not busy. Then, the computer 150 opens a TCP channel for call signaling according to Q.931 in step 420 and transmits its transport address to the computer 151, thereby requesting connection in step 430. The computer 151 transmits the admission request message ARQ to the gatekeeper 120 in order to request a connection admission. In step 450, the gatekeeper 120 transmits the admission confirm message ACF to the computer 151. After the call connection is admitted, the computer 150 transmits an H.245 transport address to the computer 151, thereby notifying completed connection in step 460. The computer 150 opens a TCP channel for H.245 in step 470 and establishes an RTP/RTCP channel for a voice or video call in real time by exchanging H.245 messages in step 480. Then, the call progresses by UDP in step 490.
The above call process has been described in the context of the ITU-T recommendations and its detailed description is omitted here.
Telephony over the IP network is viable on the premise that each terminal (computer, Internet phone, etc.) is assigned to a unique IP address. In other words, a terminal connected to a local IP network and having no unique IP address cannot receive/transmit data. When a telephone call is conducted over the IP network, each IP address functions as a telephone number. Therefore, a computer without an IP address cannot conduct a telephone call.